In EDM, a working electrode is juxtaposed with an electro-conductive workpiece across a machining gap (which may be filled with a machining liquid) and a pulsating DC or AC voltage is continuously applied to the machining gap to produce successive electrical discharges across the machining gap in order to remove material from the workpiece.
Massive stationary machine tools have employed EDM for the cutting of holes or cavities in metal workpieces (and, in particular, intricate designs) for many years, and step-wise improvements in the technology have been made from time to time.
For instance, U.S. Pat. No. 3,848,108 discloses an improved servo system for advancing or feeding the electrode in an EDM apparatus which possesses increased sensitivity and stability. U.S. Pat. No. 4,439,659 discloses an electrode servo-feed system having a quick-response electrode retraction and reciprocation. U.S. Pat. No. 4,450,337 discloses an EDM method and apparatus and claims the features of fine machined quality, stable machining performance, and a removal rate and a low wear of the tool electrode, wherein a continuous voltage (without pulsing) provides a succession of stock removal discharges. U.S. Pat. No. 4,638,141 discloses a modular EDM system which can be easily assembled to perform a given job and then can be taken apart and rearranged for the next job.
Basically, the existing EDM machine tools have a number of advantages, as follows: (1) not limited by the hardness of the workpiece, as long as this workpiece is electro-conductive; (2) accuracy and a high degree of stability of a hole-drilling process; (3) no part distortion since no vibration is applied to the workpiece; (4) no heat distortion owing to liquid cooling.
However, these existing EDM machine tools are bulky and somewhat massive, expensive, and cannot be brought to a remote job site. The latter is a major disadvantage and requires that the workpiece be brought to a shop where the EDM machine is installed. Since the workpieces sometimes encountered are heavy and cumbersome, such as construction equipment, it is impractical to bring such workpieces to the EDM equipment. This is particularly the case when the hole is to be drilled in a broken tap, bolt, drill, etc. embedded in a large piece of equipment or other workpiece.
Accordingly, it would be desirable to provide a portable, compact, relatively lightweight and easily handled EDM tool which could be brought to the remote job site to drill a hole or holes in a broken bolt (for example) in order to disintegrate the bolt and remove it from the equipment, such as a bulldozer.
Besides, most stationary EDM tools are slow, too precise for certain purposes and very expensive. A portable EDM tool which would be very fast, sufficiently precise for the special purposes intended herein, and relatively inexpensive, would be very desirable.